Hi Simon,
I don't think you'll find specific plans to do that sort of conversion. Each manufacturer or DIY enthusiast will have their own ideas on it.
Essentially you'd chop the bottom section off your solid DOB as a mirror cell base, chop the top bit off as the upper OTA with focuser and mirror and find some solid mechanicals to rejoin them without using a big tube. Most use aluminium tube rods or similar.
The hard part is making up rigid connectors and getting accurate alignment & collimation. Or if you are going down the rocker box open scope then there are all the design elements to achieve balance and rigidity etc. Having built about 5 different scopes, mostly of open truss design and various other astro hardware it really comes down to what you think you have the resources to do. My first scope build was pretty ugly and simple but it worked and I have since built several Serrurier and open framed scopes with almost scrap wood and metal from work and the home workshop.
I'd suggest searching all the astro DIY sites\threads you can find and collecting pix and drawings then deciding on a design you like and think you can build and doing your own concept. Plenty on here will help with critique and suggestions if you post up possible drawings or ask questions. You will have to do some metal work although most should be related to fixtures. Your local hardware supplier has many options and aluminium is actually quite easy to work with.
Come up with a design concept drawing or similar, post it here and we'll all get stuck in and turn it into a working scope.
(My latest build uses a pair of cut off tripod legs as the expanding truss elements. Anything is possible
)
